The formation of the TFPI-Factor Xa complex results in an inactive Factor X molecule and this bimolecular complex functions as a specific tissue factor-Factor VIIa inhibitor. An intricate interaction between tissue factor protein, native phospholipid, Ca++, Factor VII, Factor Xa and TFPI results in an inactive complex. Investigations in the laboratory are beginning which will address the ability of tissue factor/phospholipid from brain tissue from various species. It is well known that tissue thromboplastin from bovine rabbit and human sources differ in their ability to activate coagulation in recalcified plasma. This suggests that the interaction of thromboplastin with Factor VII and/ or the interaction of Factor X with this complex is an important event in the initial reactions of coagulation. Initial experiments attempted to prepare phospholipid (PL) vesicles which bound 125I-factor Xa in order to determine if the interaction of Xa with PL inhibited or enhanced TFPI binding to Xa. Other experiments attempted to demonstrate Xa binding to platelets. Platelets are the acknowledged PL source for coagulation in vivo. We had hoped to use flow cytometry and fluorescein-conjugated anti-Xa to measure the amount of Xa bound to the platelet surface. Neither of these systems provided the sensitivity necessary to characterize binding.